text: The x-axis of a Tanabe–Sugano diagram is expressed in terms of the ligand field splitting parameter, Δ, or Dq (for "differential of quanta"), divided by the Racah parameter B.  The y-axis is in terms of energy, E, also scaled by B. Three Racah parameters exist, A, B, and C, which describe various aspects of interelectronic repulsion. A is an average total interelectron repulsion. B and C correspond with individual d-electron repulsions. A is constant among d-electron configuration, and it is not necessary for calculating relative energies, hence its absence from Tanabe and Suganos studies of complex ions. C is necessary only in certain cases. B is the most important of Racahs parameters in this case. One line corresponds to each electronic state.  The bending of certain lines is due to the mixing of terms with the same symmetry. , T, etc.), but "g" and "u" subscripts are usually left off because it is understood that all the states are gerade.) labels may be written in other locations for clarity.  Term symbols (e.g. P, S, etc.) for a specific d free ion are listed, in order of increasing energy, on the y-axis of the diagram. The relative order of energies is determined using Hund's rules. For an octahedral complex, the spherical, free ion term symbols split accordingly:
  Certain Tanabe–Sugano diagrams (d, d, d, and d) also have a vertical line drawn at a specific Dq/B value, which is accompanied by a discontinuity in the slopes of the excited states energy levels.  This pucker in the lines occurs when the identity of the ground state changes, shown in the diagram below. The left depicts the relative energies of the d ion states as functions of crystal field strength (Dq), showing an intersection of the T and the E states near Dq/B' ~ 2.1. Subtracting the ground state energy produces the standard Tanabe–Sugano diagram shown on the right.
  This change in identity generally happens when the spin pairing energy, P, is equal to the ligand field splitting energy, Dq.  Complexes to the left of this line (lower Dq/B values) are high-spin, while complexes to the right (higher Dq/B values) are low-spin.  There is no low-spin or high-spin designation for d, d, or d because none of the states cross at reasonable crystal field energies.